In mammals, the transition from breastmilk to solid food plays a major role in shaping the community of gastrointestinal microbiota. The marmoset (Callithrix jacchus) can be utilized as a translational and developmental model for advancing our understanding of these gut microbiome changes across key developmental transitions. Fecal samples (N=139) were collected from six infant marmosets through five months of age, and categorized into developmental stages: Lactation Only, Lactation+Similac®, Lactation+Solid Food, and Solid Food. Samples were also collected from adults. We sequenced the V4 region of the 16s gene utilizing QIIME to evaluate bacterial diversity and abundance in the gut microbiome. Marmoset microbiomes contained a core set of operational taxonomic units (OTUs) (Actinobacteria, Bacteroidales, and Firmicutes) that were shared across all five diet and age types. In nursing infants, Bifidobacterium was the predominant taxon, constituting ~40% of the microbiome. This taxon was markedly reduced during the milk—food transition stage and returned to prominence in adulthood (~30% of adult microbiome). Infants fed supplemental formula had 50% lower bacterial diversity relative to breastfed infants, and their samples contained a high proportion of Cronobacter. The transition from breastmilk to a solid food diet alters the gut microbiome as infant marmosets mature into adulthood, and perturbations during this transition, including formula feeding, may alter healthy microbial diversity. Supported by Nebraska Food for Health Center, UNO-ORCA, and NIH-HD089147.